Control method, especially for load balancing of a plurality of electromotor drives

ABSTRACT

A load-balancing method for a multiplicity of controlled drives, especially individual electric motors of the rolls of a leveling machine for metal strip, in which all of the motors have speed and current-control circuits and are interconnected by a superordinate memory programmable computer or controller. A respective load-balancing controller, preferably of the PI type, is provided between the computer and the input to the current controller to vary the current setpoint signal supplied to the latter by the speed controller in each system.

FIELD OF THE INVENTION

Our present invention relates to a control method for load balancing ofa plurality of controlled, electromotor drives and, more particularly,for the drives of individual leveling rolls of a roll-type levelingmachine for the leveling of metal strip or the like, but also suitablefor use in other systems in controlling a plurality of electric motorswhere load balancing is desirable. The invention especially is relatedto a hierarchical system of control wherein individual controls areprovided for the respective motors, e.g. feedback control, and asuperordinate automating controller, computer or the like is providedfor the entire group of electromotors for which load balancing is to beeffected.

BACKGROUND OF THE INVENTION

When a controlled drive system is referred to herein, we mean toindicate a drive system having an electric motor which can be providedwith a feedback type of control. When a plurality of controlled drivesystems are used to distribute a driving force onto a common element, asis the case for example in a metal strip processing line, the drives forindividual rolls of a leveling machine, twin-drive roll stands,apparatus for fabricating or processing textile and paper and the like,to avoid an asymmetric load distribution upon the individual drivesystem and the detrimental effect this may have upon the drive systemsand the workpiece, it is desirable to effect a load balancing.

For example, the asymmetric load distribution can be compensated by acontrol process in which the difference between the armature currents oftwo successive drives produces a signal which is used, in turn, toreduce the speed setpoint in the speed control circuit of the morestrongly loaded drive to thereby bring about a load balancing. This formof load balancing is based upon a cascade regulation system, i.e. eachpair of drives successively along the workpiece produces such adifference signal and that difference signal is used to reduce the speedsetpoint of the more strongly loaded drive of the pair. The signal forload balancing must first be compared to an output from the slower speedcontroller and then supplied to the current controller. This has thedrawback that the load-balancing system operates with relatively highinertia, i.e. cannot respond rapidly to load fluctuations. Such aload-balancing system is not suitable for machines with high dynamicrequirements as is the case, for example, with metal strip leveling in aroller-type leveling machine wherein individual drives are provided forthe leveling rollers.

OBJECTS OF THE INVENTION

It is, therefore, the principal object of the present invention toprovide an improved load-balancing system whereby the aforedescribeddrawbacks can be avoided.

More specifically, it is an object of the invention to provide animproved load-balancing method, particularly suitable for themultiplicity of drives of the individual rolls of a metal-strip levelingmachine which can achieve load balancing with a minimum of inertia, i.e.with a significantly improved response time.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter areachieved, in accordance with the invention, in that for the separatecurrent-control and speed-control circuit of each drive, where asuperordinate controller (e.g. a programmable controller) is providedfor all of the drives, a respective load balancing controller isincluded. This can act directly upon the current controller in anadvantageous manner to enable the system to respond to the high dynamicrequirements. The additional load-balancing controller permits theactual momentum situation of each drive to be individually varied byensuring the current setpoint modification of the respective drivethrough this load-balancing controller individual to that drive. As aresult, each individual drive has its own load-balancing controllerwhich enables a predetermined load distribution over all of the resultsof a metal-strip leveling machine, for example, to be maintained aspreset. Furthermore, the load-balancing control permits not only thedesired load distribution but also the momentum overload peaks, whichresult as the leading edge of the strip is fed into the apparatus andengages the roll, to be compensated.

According to a feature of the invention, the input signal of theload-balancing controller is formed from the difference between thesignal representing the actual current I_(act) and a setpoint valueI_(comp).set, whereby the I_(comp).set is calculated from the momentumdistribution factor K_(i) and the maximum arising current I_(j). Foreach individual drive, the actual current (armature current) I_(act) ismeasured and fed to the superordinate automating unit, i.e. the commoncontroller or computer, which can be a programmable microprocessor-basedcontroller having a program and associated data stored in memory, whichcan compare that specific I_(act) value with the setpoint for the propermomentum distribution over the drive system represented by I_(comp).set,to generate a correction value I_(corr).. The current correction valueI_(corr).* is provided as an input signal to the respectiveload-balancing controller.

The setpoint value I_(comp).set is calculated from the formula: ##EQU1##where i is the index for the respective drive, i.e. represents thenumber of the drive along the workpiece path, I_(j) is the actualcurrent value of the respective drive at its maximum for maximum loadingof the particular drive when the setpoint momentum distribution isachieved and, K_(i) (≧1) is a distribution factor for the drives withreference to the drive having the greatest loading.

If a load distribution is desired in which the torques of the individualdrives are to differ along the workpiece, each of the drives will have adifferent K_(i) factor and thus each load-balancing controller will beassociated with a different setpoint value I_(comp).set.

Referring again to a leveling machine as an example, a load distributionis customarily set before the leveling process is commenced and is to bemaintained during the leveling process. For this purpose, thesuperordinate controller is programmed to output setpoint valuesI_(comp).set which can be fixed or which can be selected from tablesand/or interpolated, depending upon the particular requirements.

If it is assumed, for example and for purposes of explanation that thereare three drives, 1, 2 and 3 and, for a particular purpose that drives 1and 3 are each to receive half the load on drive 2, the computercalculates the setpoint values I_(comp).set for the drives 1 and 3 againto the above-mentioned formula so that the I_(j) corresponds to theI_(act) of the second drive and the K_(i) factor is 2. The setpointI_(comp).set for the second drive can be calculated utilizing K_(i) =1or in a corresponding calculation.

According to a further feature of the invention, the load-balancingcontroller individual to the drive and added in accordance with theprinciples of the invention is a proportional integral controller with aproportional integral transfer function. Such a PI controller has highlydynamic properties and the correction signal I_(corr)., not onlyincreases proportionally but also is integrated before the correctionsignal is applied to the current control circuit. The control responseis thereby greater so that the particular drive can be more quicklyloaded or subjected to a load reduction in response to deviation fromthe desired loading.

According to another feature of the invention, the output signal of theload-balancing controller is applied to the current regulator of therespective current-control circuit. The output signal of theload-balancing controller which can be greater, less or equal to thesetpoint current depending upon the load distribution and the actualloading can be supplied to an adder between the output of the speedcontroller and the input of the current controller. The current setpointcan derive from the speed controller.

It has been found to be advantageous, moreover, to provide each of thespeed-control circuits of the respective drives with a proportionalfeedback of the speed controller. The output signal of the speedcontroller, which by comparison to the load-balancing controller isdominant, can thereby be limited in that the output signal of the speedcontroller is fed back proportionally between 0 to 25% to the input. Theload-balancing controller thus does not have to operate at a controllimit which will allow a control deviation to remain. Indeed, thecontrol deviation on load balancing can be reduced to 0 and thereliability of the load distribution thereby enhanced by the method ofthe invention.

The method of the invention thus can comprise the steps of:

(a) providing for each of the drives a respective speed-control feedbackcircuit and a respective current-draw feedback control circuit andgenerating a respective load-responsive parameter in at least one of thecircuits indicative of loading of the respective drive;

(b) monitoring the load-responsive parameters of all of the drives in ahierarchically superordinate controller by comparing the respectiveload-responsive parameters with a setpoint value of the respectiveload-responsive parameter assigned to the respective drive and chosen tobalance loading of the drives, and generating a corrected parameter foreach drive based upon the comparison; and

(c) applying the corrected parameter through a fixed-transfer-functionload-compensating controller as an actual-value input to one of thefeedback circuits of the respective drive.

In terms of the system, the latter can comprise:

a respective speed control feedback circuit and a respectivecurrent-draw feedback control circuit for each of the drives includingmeans in at least one of the circuits for generating a respectiveload-responsive parameter indicative of loading of the respective drive;

a hierarchically superordinate controller receiving the respectiveload-responsive parameter from all of the drives for comparing therespective load-responsive parameters with setpoint values of therespective load-responsive parameter assigned to the respective driveand choosing the balance loading of the drives, and generating acorrected parameter for each drive based upon the comparison; and

a fixed-transfer-function load-compensating controller connected to thesuperordinate controller and receiving the respective correctedparameter therefrom for applying the corrected parameter as anactual-value input to one of the feedback circuits of the respectivedrive.

In the system of the invention, therefore, the speed controller, whichhas been dominant in earlier control systems, is subordinate to theprogrammable controller and the current controller is subordinate to thespeed controller.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a block diagram illustrating a control system according to theinvention; and

FIG. 2 is a diagram showing application of that system to a metal stripleveling machine.

SPECIFIC DESCRIPTION

The invention is applicable to, for example, a strip-leveling machine 20which, as shown in FIG. 2, can comprise a multiplicity of rolls 21, 22,engageable with the strip 23 from opposite sides and provided withrespective drives 24, 25, etc., each including an electric motor 1(FIG. 1) with its individual control circuits 3 and 4 and all of whichare connected in a load balancing control circuit 2 (FIG. 1) describedin greater detail hereinafter.

Each of the electric motors 1 can have a separate current controlcircuit 3 and a speed-control circuit 4. The current-control circuit 3comprises a current controller 5 which may be of the proportional orproportional-integral type and a load-balancing controller 6 which is ofthe PI type previously described.

The speed-control circuit 4 comprises a speed controller 7 which mayalso be of the PI type and a proportional feedback unit 8 connected inparallel to the controller 7.

An actual current pickup I_(act) from each motor, which can representthe respective current through the armature thereof, serves as an inputsignal for a superordinate memory programmable computer or controller 9shown as a black box which receives corresponding inputs from all of thedrives 24, 25, etc. The memory programmable controller 9 generates foreach drive a setpoint I_(comp).set which can be referred to as acompensating setpoint (selected from memory based upon previouscalculation for example or calculated as needed from data in memory)originally calculated by the aforedescribed formula and establishing thesetpoints for the desired load distribution over the entire set ofdrives.

In the computer 9, the respective setpoint values I_(comp).set arecompared with each actual current I_(act) and from the resultingcomparison, a correction current signal I_(corr). is outputted andapplied as an input to the load-balancing controller 9. Depending uponthe desired load distribution, the correction current signal I_(corr).can be smaller, greater or equal to the setpoint current for the motor1.

Because of the dynamics of the circuit, the load-balancing controller 9of each drive-control system is a proportional integral (PI) controllerso that the input signal I_(corr). is not only proportionally amplifiedbut is correspondingly integrated as well. The output signal of theload-balancing controller 6, I_(corr).* is applied to an adder 11, theoutput of which serves as the input to a current controller 5 which isalso connected to the output of the speed controller 7.

The output from the speed controller 7 is a current-setpoint signalI_(set) which is also fed to the adder 11 so that this setpoint valuecan be corrected by the correction signal I_(corr*). Thus the setpointI_(set) is increased, reduced or not modified based upon the correctionvalue I_(corr*). The resulting setpoint signal I_(set*) is thendelivered to the current controller 5 which, in the preferred case, isalso a proportional-integral controller.

The output signal I_(set) from the speed controller 7 is fed backproportionally by the controller 8 to the input of the controller 7between 0 and 25% via an adder 10 which also received an actual speedsignal N_(act) from a tachometer connected to the motor 1. An externalinput N_(set) is provided as the setpoint speed to the adder 10 as well.The result is that the output signal from the speed controller 7 islimited at the upper part of its range and the dominance of the speedcontroller 7 by comparison to the load-balancing controller 6 is reducedso that the load-balancing controller 6 does not have to face a fixedcontrol limit. The load distribution for the drive 1 can be regulatedwith a range which can extend from 0 and need not confront a fixed limitimposed upon the system by an invariable limit to the range of the speedcontroller 7.

We claim:
 1. A method of load-balancing control for a plurality ofcontrolled electromotor drives, comprising the steps of:(a) providingfor each of said drives a respective speed-control feedback circuit anda respective current-draw feedback control circuit and generating arespective load-responsive parameter in at least one of said circuitsindicative of loading of the respective drive; (b) monitoring theload-responsive parameters of all of said drives in a hierarchicallysuperordinate controller by comparing the respective load-responsiveparameters with a setpoint value of the respective load-responsiveparameter assigned to the respective drive and chosen to balance loadingof said drives, and generating a corrected parameter for each drivebased upon the comparison; and (c) applying the corrected parameterthrough a fixed-transfer-function load-compensating controller as anactual-value input to one of the feedback circuits of the respectivedrive.
 2. The method defined in claim 1 wherein an output signal fromeach of said fixed-transfer-function load-compensating controllers isfed to a current controller of the respective current-draw feedbackcontrol circuit.
 3. The method defined in claim 1 wherein said correctedparameter is subjected to proportional-integral control in saidfixed-transfer-function load-compensating controllers.
 4. The methoddefined in claim 1 wherein the corrected parameter fed as an inputsignal to each of said fixed-transfer-function load-compensatingcontrollers is a difference signal between an actual value currentsignal and a setpoint load-balancing current signal calculated from amoment-distribution factor (K_(i)) and a maximally arising current(I_(j)).
 5. The method defined in claim 1 wherein for feedback controlof speed of each drive, a speed controller is provided and is bridged bya proportional feedback loop.
 6. The method defined in claim 5 whereinan output signal from each of said fixed-transfer-functionload-compensating controllers is fed to a current controller of therespective current draw feedback control circuit.
 7. The method definedin claim 6 wherein said corrected parameter is subjected toproportional-integral control in said fixed-transfer-functionload-compensating controllers.
 8. The method defined in claim 7 whereinthe corrected parameter fed as an input signal to each of saidfixed-transfer-function load-compensating controllers is a differencesignal between an actual value current signal and a setpointload-balancing current signal calculated from a moment-distributionfactor (K_(i)) and a maximally arising current (I_(j)).
 9. Aload-balancing control system for a plurality of controlled electromotordrives comprising:a respective speed control feedback circuit and arespective current-draw feedback control circuit for each of said drivesincluding means in at least one of said circuits for generating arespective load-responsive parameter indicative of loading of therespective drive; a hierarchically superordinate controller receivingthe respective load-responsive parameter from all of said drives forcomparing the respective load-responsive parameters with setpoint valuesof the respective load-responsive parameter assigned to the respectivedrive and choosing the balance loading of said drives, and generating acorrected parameter for each drive based upon the comparison; and afixed-transfer-function load-compensating controller connected to saidsuperordinate controller and receiving the respective correctedparameter therefrom for applying the corrected parameter as anactual-value input to one of the feedback circuits of the respectivedrive.
 10. The load-balancing control system defined in claim 9 whereinsaid fixed-transfer-function load-compensating controller is aproportional-integral controller.
 11. The load-balancing control systemdefined in claim 10 wherein said superordinate controller is aprogrammable controller with a memory.
 12. The load-balancing controlsystem defined in claim 11 wherein each of said current-draw feedbackcontrol circuits includes a current controller supplying electriccurrent to a respective motor of said drive, means for tapping an actualvalue of said current from said motor and feeding said actual current tosaid superordinate controller as said load-responsive parameter, saidsuperordinate controller providing a correction current as saidcorrected parameter to the respective fixed-transfer-functionload-compensating controller and a correction current from said fixedtransfer-function load-compensating controller being fed to an adderwith a setpoint current, an output of said adder being connected to saidcurrent controller to supply an input thereto.
 13. The load-balancingcontrol system defined in claim 12 wherein each speed control feedbackcircuit includes a further adder supplied with a speed setpoint and withan actual speed value from said motor, and a speed controller connectedto said further adder and generating said setpoint current.
 14. Theload-balancing control system defined in claim 13, further comprising afeedback loop including a proportional controller and connected betweenan output of said speed controller and said further adder.